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THESES

This is to certify that the
thesis entitled

Seismotectonics of Northeast Russia
and the Okhotsk Plate

presented by

Steven Arthur Riegel

has been accepted towards fulfillment
of the requirements for

 

 

Masters degree in Geological Sciences

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0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

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SEISMOTECTONICS OF NORTHEAST RUSSIA AND THE OKHOTSK PLATE
By

Steven Arthur Riegel

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Geological Sciences

1994

ABSTRACT
SEISMOTECTONICS OF NORTHEAST RUSSIA AND THE OKHOTSK PLATE
By

Steven A. Riegel

The Sea of Okhotsk, Kamchatka, northern Japan, eastern Sakhalin, and the region
around Magadan comprise a separate microplate based on seismotectonic data in northeast
Russia. A line of teleseisms extending from the termination of the Arctic Mid-Ocean
Ridge, across northeast Russia through the Cherskii Mountains, to northern Kamchatka
defines the North American boundary with the Eurasian and Okhotsk plates. Focal .
mechanisms along this boundary indicate a change from right to left-lateral Slip as the
boundary crosses the North America-OkhotSk-Eurasia triple junction. The Eurasia-
Okhotsk boundary may follow the Burkhalinsk Fault in the Sette Daban Range, and is
more clearly defined to the south by seismicity along Sakhalin Island. Microseismicity
in northeast Russia is most heavily concentrated in the northwestern corner of the
Okhotsk plate. Two sets of faults, sub-parallel to the proposed plate boundaries, are
visible in Landsat photographs and topographic maps. The distribution of seismicity and
fault systems suggests that the boundaries between North America, Eurasia, and the

Okhotsk plate are characterized by broad (200—300 km) zones of deformation.

ACKNOWLEDGEMENTS

First and foremost, I thank Dr. Kazuya (Kaz) Fujita for an incredible amount of
guidance and support over the last two and a half years. His patience, and sometimes
impatience, are greatly appreciated. Many thanks to my parents, who were supportive of
my career choice despite their initial misgivings about future employability (You aren’t
going to end up in a museum dusting rocks, are you?). I thank Glenn Kroeger for the use
of his synthetic seismogram algorithm and Dan Hackett for his help with computing
synthetics. Many thanks to Boris Koz’min, who provided access to the archives of the
Yakut Seismic Network, to Dima Gunbin, for insightful comments on microseismicity
distributions, and to Valery Irnaev and Ludmilla lmaeva for data on the faults of northeast
Russia. Dr. David Stone, Steve Estes (thanks for the spare cards left in Yakutskl), Steve
Crumley, Dr. Leonid Parfenov, Dima Gunbin, Igor Tikhonov, and Aleksandr Larionov all
contributed to this work by hosting me and providing technical and logistical support
during my trip to Russia. Dr. F. William Cambray provided insightful discussion on the
distribution and possible kinematics northeast Russian faults, and DrS. Tom Vogel and
Michael Velbel each contributed helpful comments and suggestions. Thanks, Tom, for
pointing out that I have two looks (confused and asleep). Erin Lynch determined the
relationship between energy class (K) and mb, and Dan Hackett, David B. Cook, and

Trent Faust entered microseismicity data into the computer. Kevin Mackey did most of

iii

the drudge work tracking down mislocated epicenters. David Cook, Cindy McMullen,
and Dan Olson each performed investigations of seismicity in the region, laying the
framework for this Study. Thanks to Jeff, Ken, Wass, Kris, Jason, Wei, Mike, Sabin, Bill,
John-Paul, and everyone else in the Geology Club for comic relief. Special thanks go to
Dr. Bunsen Honeydew and the folks at Muppet Labs. Diane, our fearless librarian,
deserves extra credit for her ability to pull obscure references out of thin air. Dr. Willie
Lee, USGS Menlo Park, and Dr. John Filson, USGS Reston, were key to sorting out the
equipment and financial tangles, respectively, that I encountered leading up to my trip to
Russia. Willie Lee also gave me two weeks of training on the IASPEI software package
and the digital recording system that I installed in Batagai and Tiksi. Gail Brownell and
Mark Aaker hosted me during this training. Finally, thanks to Trina, who kept me sane
during these last four months. This work was supported by the Incorporated Research
Institutions for Seismology Joint Seismic Program (IRIS-J SP), subaward nos. 0158 and
0159, and by National Science Foundation (NSF) grants OPP 90-23580, 90-24088, and
92-24193. Additional funding was supplied by an Amoco Graduate Research
Assistantship, a Michigan State University teaching assistantship, and the Yakut Institute

of Geological Sciences.

iv

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES
INTRODUCTION
GENERAL SEISMICIT Y
FOCAL MECHANISMS AND SEISMOTECTONICS
Methodology
Northern Cherskii Mountains
Central/Southem Cherskii Mountains
Northern Kamchatka
Sette Daban/Suntar Khayata Ranges
PLATE BOUNDARIES
MICROSEISMICITY
FAULTS AND LINEAMENT S
DISCUSSION
CONCLUSIONS
APPENDIX 1

1983 June 10

1980 February 1

1963 May 20

vii

viii

13

16

25

25

31

33

37

39

43

45

49

49

52

1964 July 21

1973 December 15

1978 August 12
APPENDIX 2

BIBLIOGRAPHY

TABLE OF CONTENTS (continued)

vi

55

57

57

61

65

Table 1.

Table 2.

LIST OF TABLES
Focal mechanism data for the teleseisms of northeast Russia

Focal mechansim data for the teleseisms of the Laptev Sea
and Lena River delta

vii

10

46

Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

Figure 10.
Figure 11.

Figure 12.

Figure 13.

Figure 14.

Figure 15.

Figure 16.

LIST OF FIGURES
Overview of northeast Russia
Teleseismicity and microseismicity of northeast Russia
Focal mechanisms for teleseisms of northeast Russia
Synthetic seismograms for the event of April 19, 1962
Synthetic seismograms for the event of November 22, 1984

First motion-based focal mechanisms for events
in the northern Cherskii Mountains

Synthetic seismograms for the event of June 5, 1970

First motion-based focal mechanisms for events
in the central/southern Cherskii Mountains

Synthetic seismograms for the event of May 22, 1981
Synthetic seismograms for the event of June 19, 1970
Synthetic seismograms for the event of September 30, 1971

First motion-based focal mechanisms for events
in the Sette Daban/Suntar Khayata Ranges

Microseismicity of northeast Russia for 1976
and 1977 showing aseismic rings due to editing

Histogram Showing seasonal variations in amounts
of reported microseismicity in northeast Russia

Major faults of northeast Russia

Comparison between the Okhotsk region and the Middle East

viii

14

15

17

20

22

24

26

28

29

34

36

38

41

Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.

Figure 27.

LIST OF FIGURES (continued)
Overview of the Laptev Sea and Lena delta
Short-period synthetic seismograms for the event of June 10, 1983
Long-period synthetic seismograms for the event of June 10, 1983
Synthetic seismograms for the event of February 1, 1980
Synthetic seismograms for the event of May 20, 1963
Synthetic seismograms for the event of July 21, 1964
Synthetic seismograms for the event of December 15, 1973
Synthetic seismograms for the event of August 12, 1975
Synthetic seismograms for the event of January 21, 1976
Synthetic seismograms for the event of September 9, 1968

Synthetic seismograms for the event of May 18, 1971

ix

48

50

51

53

54

56

58

60

62

63

64

INTRODUCTION

The plate tectonic configuration of northeast Russia has been studied by numerous
workers (Chapman and Solomon, 1976; Zonenshayn et al., 1978; Savostin and Karasik,
1981; Cook et al., 1986; Parfenov et al., 1987; Fujita et al., 1990a; DeMets, 1992; and
others), with debate centering on the location of the North America-Eurasia boundary and
the possible existence of an independent plate or block containing the Sea of Okhotsk and
portions of the adjoining Asian continent. In this thesis I examine seismotectonic data
from northeast Russia to demonstrate that an independent Okhotsk plate is necessary to
explain the distribution of earthquakes in the region and the senses of morion indicated
by their focal mechanisms.

The boundary between North America (NA) and Eurasia (EU) is clearly defined in the
Arctic Ocean by a narrow band of epicenters marking the Arctic Mid-Ocean Ridge. AS
the boundary crosses onto the Asian continent near the Lena River delta (73° N, 126° E),
the band of epicenters becomes diffuse, and broadens to cover much of the Laptev Sea.
Some workers (Chapman and Solomon, 1976; DeMets, 1992) favor extending the NA-EU
boundary south across Siberia to join with the seismicity of Sakhalin Island, then on to
northern Japan, attaching the Okhotsk region to the North American plate. Others (e.g.,
Savostin and Karasik, 1981; Cook et al., 1986; Parfenov et al., 1987) prefer to have the
North American boundary follow the band of teleseisms through the Cherskii Mountains
to northern Kamchatka and suggest that the Sea of Okhotsk, Kamchatka, northern Japan,
eastern Sakhalin, and the area around Magadan comprise a separate Okhotsk microplate

or block.

2

Previous Studies of the seismicity of northeast Russia were often limited by the
availability of P-wave first motions from the ISC or Russian bulletins (e.g., Fujita et al.,
1990b). I reexamine the larger teleseisms in northeast Russia using Short-period synthetic
seismogram modelling and first motion data from records of the World Wide
Standardized Seismograph Network (WWSSN), and the Yakut and Alaska regional
networks. The combination of waveform modelling and regional first motions allows
good constraint of several new events, improvements to previous solutions, and at least
some constraint for previously unconstrained events. In addition to the teleseisms, I
examine microseisrrricity in the region for possible trends in its temporal or spatial
distribution. Finally, I use the focal mechanisms and the distribution and recent motions
of faults in northeast Russia, based on field studies and Landsat photography, to support
the existence of an independent Okhotsk (OK) plate that is being extruded to the

southeast due to the convergence of North America and Eurasia (Figure 1).

 

 

120 135 150 165 180 165
I 2 l I l l l
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— Sea tfib NA "J
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45

Figure 1. General plate configuration for northeast Russia, showing the location of the
North America (NA), Eurasia (EU), Okhotsk (OK) and Amur (AM) boundaries (Heavy
lines, dashed where inferred). Relative plate motions are shown by large arrows. Small
arrows Show the sense of slip along fault zones. 1 = Lena delta, 2 = Arctic Mid-Ocean
Ridge, 3 = Cherskii Mountains, 4 = Moma Rift, 5 = Sette Daban Range, 6 = Suntar
Khayata Range, 7 = Magadan, 8 = Shelikhov Bay, 9 = Chukotka, 10 = Koryakia, 11
Kamchatka, 12 = Sakhalin Island, 13 = Kolyma Highlands, 14 = Lena River, 15
Indigirka River, 16 = Adycha River basin.

GENERAL SEISMICITY

The Cherskii Seismic Belt (CSB) is defined by the broad zone of teleseismicity and
microseismicity extending from the termination of the Arctic Mid-Ocean Ridge in the
Laptev Sea, through the Cherskii Mountains, to the Kamchatka peninsula north of the
Aleutian-Kamchatka arc intersection. Figure 2 shows the shallow seismicity of northeast
Russia where teleseisms are Shown from 1735 to the present, and microseismicity from
1964 to 1987. Almost all seismic activity in northeast Russia is confined to the CSB; the
center of the Sea of Okhotsk, and the extreme northeastern part of Siberia (with the
exception of the far eastern portions of Koryakia and Chukotka), are essentially devoid
of shallow teleseisms.

Only a few large Shocks (April 14, 1951, M 6.5; May 18, 1971, mb (body wave
magnitude) 5.9, MS (surface wave magnitude) 6.6) have occurred in the CSB, and
teleseismicity is dominated by small to medium-sized events (mb 4.0-5.0). Most of the
larger events (m,D > 5.0) in the southern Cherskiis form a lineation along the northern edge
of the belt, extending from the epicenter of the May 18, 1971 event (6392" N, 146.10°
E) across Shelikhov Bay to northern Kamchatka. The east coast of Kamchatka, north of
the Aleutian-Kamchatka junction, is very active. A north-trending belt of events, closely
confined to the eastern coast of Kamchatka, connects the eastern end of the Cherskii
Seismic Belt with the seismicity of the Aleutian-Kamchatka junction. A possible second
lineation of epicenters extends from the central Cherskiis southward through the Suntar-
Khayata Range. The projection of this trend extends towards Sakhalin Island, and it is

along this alignment that some workers (Chapman and Solomon, 1976; DeMets, 1992)

120 135 150 165 180 165

 

 

 

 

 

 

 

Figure 2. Shallow seismicity of Northeast Asia from 1735 to 1992 (teleseisms) and 1964-
1987 (microseismicity). Teleseisms are shown by large dots; microseisms are shown by
small dots. For clarity, the seismicity of the Kuril trench, Sakhalin Island, and the
Aleutian trench has been omitted. Plate boundaries and plate name abbreviations are
shown as in Figure I. CSB = Cherskii Seismic Belt. BSB = Baikal Seismic Belt. ZYR
= Zyryanka. Data on teleseism epicenters are taken from the ISC Bulletin (1964-1993),
Preliminary Determination of Epicenters (1971-1993), Rothe (1969), Gutenberg and
Richter (1965), Materiali Po Seismichnosti Sibiri (Data on the Seismicity of Siberia)
(1979-1989), Kondorskaya and Shebalin (1982), and the Russian operational bulletin.
Microseismicity data were taken from ZemIetryaseniya V SSSR (Earthquakes of the
USSR) (1961-1987).

6

place the North America-Eurasia plate boundary. Starting in the mid-1960’s, regional
seismic networks with three-component Short-period instruments were deployed in both
the Yakut ASSR (now Sakha Republic) and the Magadan region. As a result, over 7,000
rnicroearthquakes have been catalogued from the CSB. Reported microseismicity in the
CSB is heaviest in the region northwest of Magadan, and diminishes rapidly north of the
CSB. This is not believed to be an artifact of station distribution as the opening of the
Station in Zyryanka in 1982, north of the CSB, did not alter the microseismicity
distribution (Riegel et al., 1993). Within the central Cherskiis the detection threshold
(Artamonov and Mishina, 1984), where a minimum of three stations records the event,
is energy class (K) 8 to 9 (1021 to 1022 ergs), approximately equivalent to mb 3.0 to 3.5.
In general, the detection threshold in northeast Russia is not greater than K = 11 (mb ~
4), with the exception of a gap along the EU-OK boundary between the April 14, 1951
epicenter (61.13° N, 136.57° E) and the eastward continuation of the Baikal seismic belt
(Figure 2). Due to poor coverage here, recorded microseismicity is much lower than
elsewhere, although the lack of teleseisms in the gap is notable, suggesting that slip along

this region is extremely slow.

FOCAL MECHANISMS AND SEISMOTECTONICS
Methodology

AS part of an ongoing cooperative research program between Michigan State
University, the University of Alaska-Fairbanks, the Yakut Science Center, and the
Magadan Experimental Methodological Seismological Division (EMSD) I gained acceSs
to previously unavailable Russian data. To determine focal mechanisms, a combination
of first motion data and synthetic seismogram modelling was used. First motion data
were obtained from rereading records of the WWSSN and Yakut, Magadan, and Alaska
regional networks, supplemented by first motions from the ISC and Russian operational
bulletins. Synthetic seismograms of WWSSN records were generated by forward
modelling, using the algorithm of Kroeger (1987). The final crustal model and source
parameters are those which provided a subjectively acceptable fit to the observed data.
Many of the events examined proved difficult to model, and depth phases often showed
apparent variations in depth on the order of a kilometer for the same event. This is
probably due to a combination of variable sediment thickness and reflections from
irregular topography in the source region. In areas such as the Laptev Sea, where there
is generally a more uniform, flat-lying layer of sediment, much better waveform fits have
been obtained (Cook, 1988; Fujita et al., in prep).

Previously unavailable Russian solutions for several events in the CSB (e. g., Imaev et
al., 1990) and Harvard-determined centroid moment tensor (CMT) best-fitting double
couple solutions from the literature (e.g., Dziewonski et al., 1986) are also used; the

method for inverting long-period waveform data to obtain the moment tensor solution is

8

found in Dziewonski et a1. (1981). Some of the events studied have both a CMT and
Short-period synthetic seismogram-based solution. I have found that for events where
synthetic seismograms Show very good fits to observed waveforms (e.g., 1980, February
1 and 1983, June 10; Appendix), the CMT solution is very Similar to my mechanism.
This similarity leads me to conclude that short-period synthetics can provide well-
constrained focal mechanisms for medium-Sized events, and therefore may also be applied
to small events that do not produce sufficiently well-recorded long-period waves for CMT
analysis.

Figure 3 shows all published, revised, and new focal mechanisms in the CSB. The
numbers next to each focal mechanism correspond to Table 1. When multiple solutions
are available for an event the preferred solution (Table 1) was chosen based ongthe
following ranking (from low to high weight): Bulletin reported P-wave first motions, P-
wave first motions read by the original authors, P- and SH-polarities read by the original
authors, Rayleigh-wave radiation patterns, and waveform modelling. For mechanisms
based on Similar data the more recent one is usually preferred. Taken together, these
focal mechanisms provide a clear picture of the nature of seismic slip in the CSB. I

discuss these mechanisms in detail below, beginning in the northern portion of the CSB.

l35 I40 I45 I50 I55 160 l65 I70

     

    
    

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Figure 3. Focal mechanisms for northeast Russia, demonstrating the change in slip
direction along the plate boundaries. The number next to each mechanism corresponds
to Table 1. Compressional quadrants are black; less certain mechanisms are stippled.
Dashed nodal planes indicate unconstrained solutions or ones for which no data were
presented in the original reference.

Table 1. Focal mechanism data for events shown in Figure 3. h = depth. mb = body
wave magnitude. Ms = surface wave magnitude. ST/DP/RK = Strike/Dip/Rake 0f the
fault plane using the convention of Aki and Richards (1980), p. 106. PL. 1 and PL. 2 =
Strike and Dip of the two nodal planes. Axes given as plunge/trend. Ave73 =
Aver’yanova, 1973; Cha93 = Chang et al., 1993; C0084 = Cook et al., 1984; Cor75 =
Cormier, 1975; Dzi85 = Dziewonski et al., 1985; Dzi86 = Dziewonski et al., 1986;
Dzi87a = Dziewonski et al., 1987a; Dzi87b = Dziewonski et al., 1987b; Dzi87c =
Dziewonski et al., 1987c; Dzi88b = Dziewonski et al., 1988b; Dzi90a = Dziewonski et
al., 1990a; Dzi90b = Dziewonski et al., 1990b; Dzi91 = Dziewonski et al., 1991; Ima90
= Imaev et al., 1990; K0284 = Koz’min, 1984; McM85 = McMullen, 1985; Mis67 =
Misharina, 1967; Rie93 = Riegel et al., 1993; Sta76 = Stauder and Maulchin, 1976; Vei74
= Veith, 1974; Zob77 = Zobin and Simbireva, 1977.

10

TABLE 1

 

 

No ram 110 or LAT. mu. Mia) ab 11: ST/DP/RK 91.. 1 PL. 2 T-Axrs P-Axrs DATA‘ T‘ c’ SOURCE
TIH£(UTC) (N) (B)

1 1943 03 07 58.5 166.0 30 6.5 234/65/337 234/65 334/70 03/105 33/193 8 s - Avo73
03 01 39.8

2 1945 04 15 57.17 163.71 20 6.8 356/86/005 356/86 266/85 06/221 00/311 3 s P ch85
02 35 22

3 1951 02 12 65.8 137.0 6.4 143/65/114 143/65 292/29 68/078 18/222 8? T - Imo90
17 22 00

4 1951 04 14 61.13 136.57 6.5 090/70/026 090/70 350/65 35/310 05/220 3 s - M1867
13 33

s 1952 11 30 56.41 163.15 0 7.3 176/84/010 176/84 085/80 12/041 02/311 8 s P HCHBS

6 1959 10 30 66.0 137.5 33 5.3 6 330/50/090 330/50 150/40 85/240 05/060 8 T U C0084
04 00 25.7

7 1962 04 19 69.80 138.98 12 6.2 120/40/100 120/40 287/51 05/023 82/155 PM T 0 '(LP)
23 16 04.5 137/31/115 137/31 299/60 13/036 72/183 PH T 0 '(sp)

8 1964 11 11 56.63 161.32 48 5.3 214/72/031 214/72 114/60 35/078 08/342 8? F aches
13 17 38.5

9 1964 11 11 56.68 161.22 48 5.5 223/60/023 223/60 111/70 35/070 06/164 8P 3 P HCHBS
19 06 59.2

10 1966 07 19 56.24 164.83 20 5.3 119/86/188 029/81 119/86 03/254 08/340 3? s ! Cor75
01 40 55.0

11 1968 09 09 66.17 142.13 4.9 5.0 5 250/81/025 250/81 156/66 23/116 11/021 PH 3 r Rie93
02 20 59.2

12 1969 11 22 57.700 163.719 33 6.2 7 032/75/101 032/75 172/19 58/319 29/112 PS T c Sta76
23 09 39.0

13 1969 12 02 57.44 163.43 9 5.0 4 207/74/222 207/74 113/50 15/340 40/085 B N - Zob77
04 12 31

14 1969 12 08 57.01 162.23 54 4.7 5 045/80/126 045/80 148/38 43/350 26/108 8 T - Zob77
05 18 33.1

15 1969 12 23 57.32 163.10 33 5.4 5 033/82/049 033/82 295/42 38/266 25/154 3? T r “cuss
13 22 58.7

16 1970 01 27 57.607 163.616 46 5.0 4 010/45/090 010/45 190/45 90/280 00/100 3? T - Vei74
11 17 32.2

17 1970 06 05 63.26 146.18 2.2 5.4 5 212/79/168 212/79 305/78 17/168 01/259 PH s G '
10 31 53.9

18 1970 06 19 57.38 163.35 33 5.2 5 010/64/120 010/64 317/40 58/326 13/078 PH T r '
18 52 33.8

19 1971 05 18 63.92 146.10 13 5.9 305/82/006 305/82 214/84 10/170 01/260 PH 9 c R1693
22 44 43.8

20 1971 09 30 61.61 140.4 2.1 5.5 241/89/181 241/89 150/89 01/016 01/106 an s G '
21 31 26.1

21 1972 01 13 61.94 147.04 33 5.3 5 100/88/008 100/88 009/82 06/324 04/234 BPR s F ucHBS
17 24 23.2

22 1972 08 03 59.51 163.10 6 5.2 4 278/74/340 278/74 014/70 03/328 26/236 398 s P “cuss
12 36 46.9

23 1974 06 19 63.14 150.92 15 4.9 4 340/77/000 340/77 070/90 09/025 09/117 3? s P '
03 09 36

24 1975 11 04 60.02 160.32 52 4.7 324/57/090 324/57 144/33 78/234 12/054 8? T F K0284
12 41 12.4

25 1976 01 21 67.73 140.03 10.5 5.0 4 095/60/012 095/60 359/80 28/313 15/051 B?” s c 81693
06 01 48.5

26 1976 01 21 58.92 163.55 7 5.4 5 210/74/343 210/74 304/73 00/258 24/168 an s 7 genes
18 02 04

27 1976 01 22 58.92 163.75 42 5.2 5 184/60/352 184/60 278/83 16/047 26/145 an s P HcH85
06 35 16.2

28 1977 02 17 58.86 163.87 18 5.1 188/72/339 188/72 284/70 01/237 27/146 8? s F ch85
13 32 31.7

29 1977 11 18 60.05 143.32 33 4.7 106/20/348 106/20 207/86 38/315 45/097 3? s - Ima90
21 55 39.4

30 1978 06 05 60.15 160.30 15.0 5.1 4 006/26/119 006/26 155/67 65/042 21/255 C T G 02187b
07 05 50.9

31 1979 08 19 61.00 158.36 15 5.1 5 185/70/194 185/70 090/77 06/139 24/047 c s G 02187c
07 10 10.6

32 1979 10 07 64.932 143.768 6 4.8 4 320/48/118 320/48 101/50 70/304 01/210 3? T - 1ma90
01 29 26

11

Table 1 (continued)

33 1981 05 22 61.09 156.68 6 5.1 4.5 278/45/071 278/45 124/48 77/107 01/210 PM T F '
04 59 25.9

34 1981 08 29 65.49 136.43 9 4.7 142/58/174 142/58 235/85 26/104 19/004 8? s - 1ma90
22 23 51.5

35 1981 11 08 61.51 152.54 15.0 5.6 5.1 127/36/020 127/36 020/78 45/325 25/084 C s G D2188b
21 56 14.5

36 1982 09 03 66.82 132.8 33 4.5 117/72/191 117/72 023/80 05/072 20/339 3? S - Ima90
07 29 22.2

37 1983 03 25 63.63 149.74 16 4.7 113/69/135 113/69 222/49 46/070 11/172 BP T - Ima90
10 36 57

38 1984 08 02 60.89 144.7 33 4.9 332/90/180 332/90 242/90 00/287 00/197 BP 3 P '
21 25 37.8

39 1984 11 22 68.52 140.82 22 5.3 5.6 062/79/032 062/79 326/58 30/281 14/190 PM 5 P '
13 52 55.4 26.1 087/75/047 087/75 341/45 43/316 18/208 C T G 02185

40 1984 12 02 63.46 150.52 44 5.2 5.5 204/60/090 204/60 024/30 75/114 15/294 5 T P '
08 35 48

41 1985 01 29 64.29 145.7 33 4.5 025/45/090 025/45 205/45 90/180 00/115 8P T P '
00 36 09.4

42 1985 06 02 64.8 144.0 33 4.3 150/60/012 150/60 050/75 32/007 10/103 8P 3 P ‘
04 08 12.8

43 1985 06 24 65.25 144.49 33 4.7 081/82/312 081/82 172/44 25/139 38/028 8? N P '
03 54 32.4

44 1985 07 29 56.37 164.82 23 5.2 5.2 304/72/183 304/72 213/87 10/260 15/167 C S G 02186
06 32 19.7

45 1986 01 01 56.15 164.76 28.0 5.6 4.7 293/30/108 293/30 092/62 71/338 16/189 C T 6 021878
22 10 20.3

46 1986 12 18 61.60 145.54 0 4.6 117/72/332 117/72 216/64 05/167 32/074 8? s F ‘
18 04 10.2

47 1987 02 11 62.955 156.623 5 4.4 4.9 094/87/090 094/87 274/03 48/004 42/184 3 T G '
00 58 20.8

48 1989 01 27 56.54 164.05 23.5 5.6 6.3 206/80/355 206/80 297/85 04/071 10/162 C s 6 Dzi90a
08 34 56.7

49 1989 04 09 59.92 145.23 10 4.8 4.3 000/68/158 000/68 098/70 30/320 02/230 BP 5 P '
04 16 25

50 1989 04 27 56.53 164.14 34.8 4.9 5.0 281/62/167 281/62 017/79 28/242 11/146 C S G Dzi90b
10 39 40.6

51 1989 05 24 56.42 164.05 43.6 5.8 6.3 018/80/007 018/80 286/83 12/242 02/332 C s G Dzi90b
13 31 20.4

52 1989 05 24 56.42 164.09 43.0 5.4 5.6 017/76/003 017/76 286/87 12/241 08/332 C s c Dzi90b
15 43 38.2

53 1990 06 25 56.38 164.45 28.4 4.9 5.0 031/86/356 031/86 122/86 00/256 06/346 C s G 02191
07 19 06.9

54 1990 11 02 65.239 145.920 10 4.6 288/80/221 288/80 191/50 19/054 35/157 8 s P '
21 54 04.4

55 1992 09 13 61.804 153.932 21 4.7 4.3 307/90/000 307/90 217/90 00/172 00/082 C s - Cha93
21 42 57.2

‘ B u Bulletin reported first notion. P s Reread first notions, R = Raleigh wave radiations. M = synthetic

seismograms. 1 s P and SH inversions, C a Centroid moment tensor. - = Unknown data type.

’ Mechanism type: T : Thrust. N 2 Normal. S = Strike-slip.
’ Constraint: 6 x Good. P 3 Pair. P 8 Poor. - a Unconstrained or no data presented in original citation.
' z This study

Dip direction is to the right as the observer faces the direction of strike.

12

13

Northern C herskii Mountains

Four events in the northern Cherskii Range were sufficiently well recorded to model.
I modelled both Short and long period records for the April 19, 1962 event (Figure 4),
resulting in two slightly different focal mechanisms (Table 1), both indicating nearly pure
reverse faulting. A source time function using two short pulses with a delay of 2.2
seconds between them was used, suggesting a complex rupture process. The epicenter
is located near the north-verging Polousnyi thrust (Imaev et al., 1990), suggesting Slip on
the more shallow, south-dipping plane.

The November 22, 1984 event (Figure 5) was extremely difficult to model, and my
synthetics are of only moderate quality. Regional first motion data, however, provide fair
constraint for a dominantly strike-slip mechanism. A CMT double couple is available
(Dziewonski et al., 1985) which is similar to my solution, although with a greater thrust
component.

Riegel et a1. (1993) determined mechanisms for the January 21, 1976 and September
9, 1968 events, indicating right-lateral slip along north trending nodal planes. This is
preferred due to the alignment of these planes with both the general trend of seismicity
in the region and the adjacent grabens 0f the Moma Rift system. Fujita et al. (1990a)
suggest that the faults bounding Moma Rift, which Opened in the Cenozoic (Eocene to
Pleistocene (1’)), have recently (1-2 Ma) been reactivated by compressive Stresses, as
shown by focal mechanisms and folds and thrusts in sediments as young as Pleistocene
(Imaev and Grinenko, 1989), although Imaev and Grinenko (1989) suggest that Slip is

taken up on newly developed low-angle thrusts, rather than on the Older system.

1962 O4 19

ALE lpz TOL lpz

MAL lpz GOL spz

M W MECHANISM:
. lpz 120 40 100 (solid)
W spz 137 31 115 (dashed)
DEPTH: 12 km
1 A SOURCE TIME:

30 SECONDS 0.2 0.0 0.2
STU SP2 1.3 2.2 0.2 0.1 0.2
CRUST:
6.0 3.5 2.7 30.0

WW 8.0 4.6 3.3
W or 3 p T
)tvaAxAAM/m/WW

 

 

Figure 4. Synthetic seismograms for the April 19, 1962 event in the northern Cherskii
Mountains. For each station the top trace is the observed record, with the synthetic
seismogram below. Closed symbols denote compressions, open symbols are dilatations.
Triangles are used for Stations that were modelled, but for which no first motion was
picked. Source time is given as rise/flat/fall in seconds. or = P-wave velocity (km/sec),
B = S-wave velocity, p = density (g/cm3), T = layer thickness (km).

14

1984 11 22

DAG spz YKC spz

 

I WWI/VH4»
WWW MECHANISM:
58 80 40 (SOLID, SYNTH)
IV 87 75 47 (DASHED, CMT)
DEPTH: 13 km

SOURCE TIME: 0.5 0.5 0.5
CRUST:

 

KOD spz NDI spz
5.8 3.4 2.4 5.0
6.5 3.8 2.7 35.0
”W 8.0 4.6 3.3
«14 MW a 6 p T
”WM “’M’\KWVVVWV"‘_V'
L A
30 SECONDS

Figure 5. Synthetic seismograms for the November 11, 1984 event. Symbols are as in
Figure 4.

15

16

West of the 1968 epicenter are four less well-constrained events. The October 30,
1959 event is a thrust with unconstrained nodal planes, based on bulletin-reported first
motions (Cook et al., 1986), and Imaev et al. (1990) report a very similar mechanism for
the February 12, 1951 event (Table 1). The strikes of the nodal planes parallel several
nearby southwest-verging thrust faults, suggesting slip on the northeast-dipping planes.
The remaining two events are dominantly strike-slip (Imaev et al., 1990). The nodal
planes of the September 3, 1982 event do not closely follow any nearby faults, although
right-lateral slip on the northwest trending plane is consisrent with other focal mechanisms
to the east in the northern Cherskiis. I suggest right-lateral slip along the southeast-
striking plane for the August 29, 1981 event following a nearby fault mapped by Imaev

et al. (1990), although they map the fault as a thrust based on field observations.

Central/Southern C herskii Mountains

Five events are clustered around 65° N, 145° E, south and east of the 1968 epicenter.
Their relatively small size leads to poor degrees of constraint for their focal mechanisms.
Two of these, October 7, 1979 (Imaev et al., 1990), and January 29, 1985 (Figure 6), are
thrusts. My first motion data suggest that the other events are dominantly strike-slip
(Figure 6). The June 24, 1985 event lies at the end of an east-trending fault, suggesfing
left-lateral Slip along the east-trending nodal plane. The June 2, 1985 and November 2,
1990 mechanisms, however, suggest left-lateral slip along the more north-striking planes.

The May 18, 1971 shock is the largest event in the Cherskii Mountains and is very

well constrained by both first motions and synthetic seismograms (Riegel et al., 1993).

Figure 6. Focal mechanisms for events in the northern Cherskii Mountains, based on
reread and bulletin-reported first motions. Circles are used for reread stations of the
WWSSN, Yakut, and Alaska networks. Squares are bulletin-reported data. Open symbols
represent dilatations, closed symbols are compressions.

17

.m ousmflm

m. Om Om: N_m mm 50 Om mv 0N0

 

NO mo mwQ tom 00 mm@ mm _0 mm@

19

Left-lateral slip occurred along the northwest trending plane as determined by directivity
studies (Filson and Frasier, 1972) and aftershock distributions (Koz’min, 1984). The
reversal of Slip direction relative to the 1968 event is immediately apparent, and this
epicenter marks the initiation of well-constrained left-lateral Slip in the CSB. South of
the 1971 event, Slightly off the main trend of epicenters in the southern Cherskiis, is the
June 5, 1970 epicenter. My short-period synthetic seismograms (Figure 7) fit this event
quite well, and the resulting mechanism is nearly identical to the 1971 event. The
synthetic for one station (CHG), however, is almost exactly 180° out of phase with the
observed record. Station polarity was checked against the calibration pulses on the record
and first motions from teleseisms, and was found to be normal. The reason for the
discrepancy is unknown, but may be due to phasing caused by reflections from irregular
topography in the source region. To the south, the January 13, 1972 event has fair
constraint based on first motions and Rayleigh wave radiation patterns (McMullen, 1985).
This event lies on an east-west lineation in topography, suggesting left-lateral slip on the
east-striking plane.

Continuing easrward, Imaev et al. (1990) report two thrusts. March 25, 1983 and
December 2, 1984, with P-axes indicating north-south compression. Inconsistent first
motions and the small size of the 1983 event lead me to assign the mechanism an
uncertain reliability, and I therefore place a low weight on this solution. The 1984 event
is constrained by ISC-reported first motions to have a large thrust component, but the data
do not constrain the nodal planes. Regional bulletin-reported first motions are

inconsistent with the mechanism of Imaev et al. (1990). and suggest a more northeasterly

1970 06 05

ALQ spz GSC spz
GOL
‘ ALQ
JWW % GSC
GOL spz UME spz
11.
5151515th51555
] MECHANISM: 212 79 168
ll DEPTH: 2.2 km
.1151ny {1... SOURCE TIME:
ill it 0.2 0.0 0.1
0.8 0.8 0.3 0.0 0.1
CRUST:
NDI 5‘” CHG SP2 3.5 2.10 2.0 0.2
h 4.6 2.70 2.5 3.6
2115-1311511 5.6 3.29 2.7 10.0

I

30 SECONDS

5581340 213201)
(351185139

oerT

Figure 7. Synthetic seismograms for the June 5, 1970 event. Symbols are as in Figure
4. The cause of the apparent polarity reversal for CHG is unknown.

20

21

trend for the planes (Figure 8). Koz’min (1984) previously Studied the nearby June 19,
1974 event using bulletin-reported first motions. I reexamined this event, adding first
motions reread from WWSSN and Yakut network records (Figure 8), and assign left-
lateral Slip along the northwest-Striking plane to follow the Ulakhan Fault (Figure 15).
CMT solutions are available for four events in the southern Cherskiis. Three of these
are part of the lineation of events between the May 18, 1971 epicenter and northern
Kamchatka. The mechanisms for the September 13, 1992 and August 19, 1979 events
both suggest left-lateral slip along east/southeast-Striking planes, parallel to this lineation.
The June 5, 1978 event is a nearly pure reverse mechanism with north-trending planes.
The November 8, 1981 event lies between the In’yali-Debin and Ulakhan faults (Figure
15), and its mechanism also suggests left-lateral slip along the southeast-striking plane,
parallel to the faults. Three additional events in the region are all thrusts. The May 22,
1981 event, constrained by me using first motions and synthetic seismograms (Figure 9),
and the November 4, 1975 event (Koz’min, 1984) both lie on the lineation of epicenters
in Shelikov Bay. The 1987 February 11 event is well constrained by first motions as a
reverse fault with a nearly vertical east-striking plane (Figure 8), although the event is
somewhat unusual in that it is located nearly three degrees north of the main line of
epicenters. The epicenter lies along the westward projection of four en echelon east-
striking lineaments north of Shelikhov Bay (Figure 15), suggesting that the east-striking

plane is the fault plane.

Figure 8. First motion-based focal mechanisms for events in the cenual Cherskiis.
Symbols are as in Figure 6.

22

.m whomflm

Om km 000 00 NA. O¢m om Ow vow

23

 

 

__ NO N@@ 9 mo $12 NO m_ 0mm:

1981 05 22

 

 

 

 

DAG spz JCT spz
w- W ;
W ”IMP” JCT ‘
ALQ
ALQ spz
MECHANISM: 278 45 071
4116 m DEPTH: 6 km
W SOURCE TIME: 0.6 0.0 0.3
CRUST:
2.5 1.4 2.1 0.5
I l 4.0 2.2 2.3 0.5
30 SECONDS 5.0 2.8 2.5 1.0

6.4 3.7 2.9
or 8 p T

Figure 9. Synthetic seismograms for the May 22, 1981 event. Symbols are as in Figure
4.

24

25

Northern Kamchatka

The August 3, 1972 event lies at the end of the Cherskii Seismic Belt in northern
Kamchatka, and marks the eastem-most expression of left-lateral Slip in the CSB.
McMullen (1985) used bulletin-reported and reread fust motions, as well as Rayleigh
wave radiation patterns, to constrain this event as left-lateral Strike-slip; the mechanism
is not well constrained, however, due to poorly recorded first motions at most stations.
A reexamination of WWSSN and Yakut short-period records produced no significant
change to the solution.

The east coast of Kamchatka, north of the Aleutian-Kamchatka trench junction, is
dominated by east-west directed thrusting (Corrnier. 1975; Stauder and Mualchin, 1976;
Zobin and Simbireva, 1977; Daughton, 1990) although a few poorly constrained Strike-slip
mechanisms are reported (Corrnier, 1975; Zobin and Simbireva, 1977). I modelled the
June 19, 1970 event (Figure 10), and my synthetics Show good fits to the data, although
the lack of stations near the nodal planes allowed a roughly 20 degree variation in the
strike of the planes. Near the intersection of the Kamchatka and Aleutian arcs numerous
right-lateral strike-slip mechanisms are reported (e.g., Newberry et al., 1986; Dziewonski

et al., 1986), most likely reflecting Pacific-North America slip.

Sette Daban/Suntar K hayata Ranges
In the Sette Daban and Suntar Khayata Ranges only six focal mechanisms are
available. The largest event, April 14, 1951, iS poorly constrained based on a few

bulletin-reported first motions and four Russian first motions (Misharina, 1967).

1970 08 19

 

 

UME spz NDI spz
W W
«M:- «MW—~—
DUG spz JER spz

3“\ ’VVV\F~

‘ MECHANISM: 010 64 120
a DEPTH: 10 km

ll-Nll '° ‘ 11W“ ' SOURCE TIME: 0.5 0.4 0.3

CRUST:
l J 3.5 2.10 2.0 1.0

NUR spz 30 SECONDS 4-0 2:35 2-3 1-5
5.6 3.29 2.7 10.0
5.8 3.40 2.8 20.0
6.5 3.85 2.9

JUA'JW’V‘ a 5 ,0 T

 

 

Figure 10. Synthetic seismograms for the June 19, 1970 event. Symbols are as in Figure
4.

27

Misharina (1967) place this event on the Burkhalinsk Fault, a major Mesozoic thrust
reactivated in modern times as right-lateral Strike-slip (Prokop’ev, 1989). A relocation
of the event places it about a degree farther to the west, however. At face value the
mechanism suggests right-lateral slip along a north-south trending plane. The September
30, 1971 event is very well constrained by regional first motions (Koz’min, 1984) and by
synthetic seismograms (Figure 11), and falls along a lineation in topography which
suggests right-lateral slip along a northeast-Striking plane.

Based on P-wave first motions, the December 18, 1986 event (Figure 12) has fair
constraint and Shows strike-slip with a component of transtension. The August 2, 1984
event (Figure 12) is poorly constrained, although first motions constrain one plane to
Strike northwest with a Steep dip. The April 9, 1989 event (Gunbina et al., 1991) has the
most data available, although many of the regional first motions used in their Study are
inconsistent. I reread first motions for Six Yakut network Stations, and use these data,
first motions from Gunbina et a1. (1991), and ISC-reported first motions to adopt a
transpressive mechanism with a north-striking plane (Figure 12). I choose the more
north-striking planes as the slip plane for each event due to the Similar Strikes of a nearby
faults, yielding right-lateral transpression. Imaev et a1. (1990) constrain the November

18, 1977 event as Strike-Slip with a component of transtension.

 

NDI spz

’J‘ I ’51 5“: V"!

1* l

‘_. d
- 12.-.—

 

Mir/\-
u 1.‘

NUR spz

 

1971 09 30

KOD spz

 

~10:
j.

QUE spz

 

4144va
JR»

 

 

30 SECONDS

 

MECHANISM: 240.6 89.2 181
DEPTH: 2.1 km

SOURCE TIME: 0.1 0.1 0.1
CRUST:

2.5 1.4 2.1 0.5

4.0 2.2 2.3 0.5

6.4 3.7 2.9

aBpT

Figure 11. Synthetic seismograms for the September 30, 1971 event. Symbols are as in

Figure 4.

Figure 12. First motion-based focal mechanisms for events in the Sette Daban range.
Symbols are as in Figure 6.

29

mm_mw oo

 

mo 00 mmm:

mmm NN n:

m_ N_ me

.NH musmflm

Om_om Nmm

30

NO mo 0mg

31
PLATE BOUNDARIES

DeMets (1992) examined slip vectors along the Kamchatka, Kurile, and northern Japan
trenches to test whether they best represent Pacific-Eurasia, Pacific-North America, or
Pacific-Okhotsk motion, and concluded that while the data are inconsistent with Pacific-
Eurasia slip, Pacific-Okhotsk motion is indistinguishable from Pacific-North America; an
independent Okhotsk plate is therefore considered to be unnecessary. However, seismicity
distributions and my mechanisms clearly demonstrate that plate configurations which do
not include a separate Okhotsk Plate are inconsistent with the data. Attaching the
Okhotsk region to North America (Demets, 1992) does not account for the disrinct
lineation of epicenters in the southern Cherskii Mountains and Shelikov Bay, nor the
seismicity north of the Aleutian-Kamchatka junction. Attaching Okhotsk to Eurasia
(Chapman and Solomon, 1976, Figure 4b.) can account for the lineation of epicenters in
the southern Cherskiis, but does not explain the change in Slip direction in the central part
of the range. All of the observed epicentral distributions and senses of motion can be
explained only by including a separate plate or block encompassing the Okhotsk region.
The center of the Sea of Okhotsk is essentially aseismic at shallow depths, further
supporting the existence of a rigid plate bounded by zones of elevated seismicity on its
margins.

Seismicity in the CSB is divided into four distinct zones based on focal mechanisms:
1) the northern Cherskii Mountains, 2) the southern Cherskii Mountains, Kolyma
Highlands, and Shelikhov Bay, 3) northem Kamchatka, and 4) the Sette Daban/Suntar-

Khayata zone. The four zones define the North America-Eurasia (1), North America-

32
Okhotsk (2 and 3), and Eurasia-Okhotsk (4) boundaries (Figure 1). Focal mechanisms

(Figure 3) clearly Show that the North America-Eurasia boundary is experiencing right-
lateral transpression. The North America-Okhotsk boundary is dominated by left-lateral
slip and compression in the southern Cherskiis and Shelikov Bay, and by east-west
compression between North America and Okhotsk along northern Kamchatka. The
Eurasia-Okhotsk boundary is characterized by right-lateral slip. I propose that the
Burkhalinsk Fault marks the eastward limit of the rigid Eurasian plate, although much of
the active slip along the boundary may be occurring along the Ketanda fault system to
the east (Figure 15). Farther to the south, along Sakhalin Island, the western edge of the
Okhotsk plate is more clearly defined by teleseismic activity. Savostin and Baranov
(1981) and Jolivet et al. (1992) demonstrate dextral slip along the length of the island by
means of focal mechanism analysis, while Chapman and Solomon (1976) report some
thrust focal mechanisms. Jolivet et al. (1992) also report structural evidence in the form
of en echelon folds and horizontal striations on faults in central Sakhalin Island, both of
which Show evidence of dextral motion.

The Baikal seismic belt extends from Lake Baikal east and northward to near the
northern end of Sakhalin Island. Motion along this belt is characterized by normal
faulting and left-lateral Slip (Sherman, 1978), and Tapponier et al. (1982) propose the
existence of an Amur block which is being extruded to the east, resulting in the Baikal
rift and associated seismic zones. The combination of thrusting and right-lateral slip
along Sakhalin is consistent with the extrusion of Okhotsk to the southeast and east-west

Shortening between Okhotsk and the Amur Block.

33

Seno and Sakurai (1993) also examine slip vectors from the Japan, Kurile, and
Kamchatka trenches. Contrary to DeMets (1992), they conclude that Okhotsk motion can
be discerned from North America. Their North America-Okhotsk Euler pole (26.4° N,
73.6° E, 0.09 deg/my) results in 0.95 chr of left-lateral Slip in the southern Cherskii

Mountains, in good agreement with the seismic data presented here.

MICROSEISMICITY

Over 7,000 microseisms are reported in ZemIetryaseniya v SSSR (Earthquakes of the
USSR) for northeast Russia during the period from 1964 to 1987. The majority of these
occur in the northwest corner of the Okhotsk Plate, and I believe them to reflect the
deformation of the plate as it being extruded. Extensive mining occurs throughout this
region, however, raising the possibility of contamination of the catalogs by mining blasts.
Large mining blasts may be confused with natural microseisms, and some Stations (e.g.,
Iul’tin) do not attempt to discriminate between them at all, resulting in the inclusion of
mining blasts in the catalogs (D. V. Gunbin, pers. comm., 1993). Plots of microseismicity
for individual years, especially during the early to mid-1970’s, reveal rings with
essentially aseismic centers and a radius of roughly 125 kilometers (Figure 13),
representing attempts to filter mining blasts from the catalogs by eliminating all events
inside of this radius of known anthropogenic sources (D. V. Gunbin, pers. comm, 1993).
If mining blasts are not completely filtered from the data, during the winter months, when
the ground is frozen and more blasting is needed to break up the deposits for Strip mining,

reported microseismicity levels Should increase. By late summer the ground has thawed

 

64

 

e e
e e . e
e e$e' .0 .
—. . .9 e . .e. ...e. 60
.9 e.e e e
e 'e
s ,- .
e e

 

 

 

Figure 13. Microseismicity in northeast Russia for 1976 and 1977. Note the well-
developed aseismic rings near (Susuman (SUUS) (62.77° N, 148.13° E) and Seimchan
(SEY) (62.92° N, 152.42° E). The rings are the result of an attempt by these Stations to
remove anthropogenic sources from the catalogs prior to publication by excluding events
within a certain distance from some settlements and mines.

34

35

sufficiently to allow strip mining without the need for blasting, and reported seismicity
Should decline.

A histogram was constructed to identify seasonal trends in the level of microseismicity
(Figure 14); monthly totals were determined for the period from January, 1970, to
December, 1987. The highest total numbers of events occurred during the months of
January through April, with cumulative totals around 600 events per month over the entire
study period. Beginning in May, however, a decline in the monthly totals is observed,
reaching a low of 393 events in September. From here the totals begin to rise again,
reaching 541 in December. The seasonal variation, though less pronounced for recent
years, is observed when the data for 1970-1979 and 1980-1987 are plotted separately.
D. V. Gunbin (pers. comm, 1993) confirms that the seasonal variation is anthropogenic,
and reflects the varying level of mine-related blasting during the year.

If the apparent seismicity of the region around Susuman is dominated by mining
activity, what does this imply for other regions of high microseismicity? The distribution
of microseismicity appears to be relatively constant from month to month, with the
exception of the Susuman region. This suggests that elsewhere the distribution is either
of tectonic origin and is the manifestation of compression of the Okhotsk plate between
North America and Eurasia, or that anthropogenic sources in other regions are more
constant throughout the year. The extremely low population density of northeast Russia
suggests that the former is the case.

Northeast of Magadan, centered at approximately 61° N, 151° E, is a region of low

seismicity. This region is bounded by the teleseisms marking the NA-OK and EU-OK

Microseismicity of Northeast Siberia, 1970-1987
1200

 

I 1980—1987
1000 ~ @1970—1979 -

1970-1987
800 - _

  
    

600 —

  
       
      

 
     
   
  
   
    

 
  

400 —

   

  

:7////////A

.7
02

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10’

VII/ll.
.:'////A

:3

  
  
  
     
    

     
    
  

V

VIII/Ill.

O

  
      
 

'6

S’l/l/l/IA

.I'///////.

:3

VII/ll.
{VII/Ill.

A A.‘

O
.9

   

4
O
5.53

  

O

  

V
3°
'0'

  

32'
20’

 

    
     
  

e
A

     

.0

    

  
 
  

         
  
   
  
  
 

  

$23M

0
A

   

0'0'4
0

e
5..
e

  

200 -

     

    

          

-_ 8.,
{i

 

 

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Figure 14. Histogram showing seasonal variation in the number of microseisms reported
in ZemIetryaseniya V SSSR for northeast Russia. Each bin represents the cumulative
monthly total during the interval 1970-1987. Note the pronounced decrease in reported
microseismicity during the months of September and October. The variation represents
seasonal fluctuations in the amount of mine-related blasting being reported in the catalogs.

36

37

boundaries to the north and west, and on the southeast by a zone of teleseisms trending
along the Chelomdzha-Yama Fault (Figure 15); both teleseismic and microseismic activity
are reduced within the zone. The lack of microseisms here is probably not an artifact of
station distribution given the coverage of the Magadan regional network (Artamonov and
Mishina, 1984). Jackson and McKenzie (1988) note that within zones of continental
deformation it is possible to have blocks of crust that act as relatively rigid, and therefore

aseismic, units. I suggest this to be the case here.

FAULTS AND LINEATIONS

Imaev et al. (1990) and Imaev et al. (1993) map an extensive system of Strike Slip and
thrust faults throughout northeast Russia, to which I add possible additional faults, based
on Strongly developed lineations seen in topography and on Landsat and Space Shuttle
photos (Figure 15). Two primary fault systems are identified, the best developed set
parallels the Ulakhan Fault, trending roughly northwest-southeast along the Strike of the
southern Cherskii Mountains. The second set strikes north-south in the Sette Daban
Range, parallel to the Burkhalinsk Fault. Both fault sets contain shorter lineations and
mapped faults that cut obliquely across the larger faults; in the northern set they Strike
generally eastward and southeastward, while in the western set they Strike northeastward.
The two fault sets, including the smaller faults and lineations, are near mirror images of
each other, suggesting that the same processes are controlling crustal deformation in both
cases. I suggest that many of the larger faults may be reactivations of the original

terrane-bounding sutures formed during the Mesozoic accretionary episodes. Some of

I45 I50 l55 I60
I

 

68

66

64

......

""""" 62

 

 

   

 
 

 

 

SEA OF OKHOTSK
l l

.fifliKH/ttm SK ‘

 

 

 

Figure 15. Major faults of northeast Russia (solid), after Imaev et al. (1990), and
Strongly-developed lineaments visible on Landsat photos and topographic maps (dashed).
Dots represent earthquake epicenters for which focal mechanisms are shown in Figure 3.
Arrows indicate sense of slip along the fault. Fault abbreviationa: U = Ulakhan, D =
Darpir, I-D = In’yali-Debin, Ch-Yu = Chai-Yureinsk, Ch-Y = Chelomdzha-Yama, Ul =

Ul’bei, Kt = Ketanda.

38

39

these may have also been reactivated as normal faults during the Cenozoic formation of
the Moma Rift system, and again at present as Strike-slip. Present-day activity on these
faults is demonstrated by epicentral locations for many of the larger teleseisms, by stream
offsets, and by seismic dislocations in the form of scarps and landslides throughout the
CSB (Imaev et al., 1990; Imaev et al., 1992).

The number of faults and distribution of seismicity along the NA-OK and EU-OK
boundaries suggest that relative plate motions are being accommodated by wide bands of
deformation consisting of several smaller blocks and slivers of crust bounded by the
larger, more rigid plates. Therefore, it is probably more correct to discuss a boundary
zone between the plates rather than a discrete boundary. Within these zones, however,
Slip may be concentrated along an individual fault, as determined by epicenter locations.
In the southern Cherskiis, the Ulakhan Fault appears to be the most active strand, with
several of the larger events located on or near it.

Along the Eurasia-Okhotsk boundary, the Burkhalinsk and Ketanda Fault systems are
the most prominent in the topography. The active seismicity, however, appears to be
concentrated to the east of the Ketanda system. In the northwestern Sea of Okhotsk,
examination and relocation of earthquakes suggest that weak activity iS concentrated along

an extension of the Ketanda fault and may strike into the seismicity of Sakhalin Island.

DISCUSSION
The NUVEL-l NA-EU Euler pole predicts that the North America-Eurasia boundary

from the Lena delta to the central Cherskiis should be in active extension. Earlier

40

geologic studies within the Moma Rift system (e. g., Grachev, 1982; Artemov and lvanov,
1988) supported this. Focal mechanisms for earthquakes in the Cherskiis, however,
clearly indicate thrusting and transpressional Strike-slip, and I prefer instead the NA-EU
Euler pole of Cook et al. (1986) (71.2° N, 132.1° E). Imaev and Grinenko (1989) note
the occurrence of folds and thrusts in Late Cenozoic sediments in the northern portion of
the system, near the Lena River delta, where they map a low-angle thrust of several tens
of meters placing Eocene coal-bearing deposits over Middle(?) to Upper Pleistocene
gravels. Evidence of compression in the region is also given by Triassic sediments that
are overthrust onto Pleistocene deposits in the Adycha River basin (Imaev et al., 1990).
On the basis of their investigations, Imaev and Grinenko (1989) suggest the onset of
compression in the region as occurring in the Middle Pleistocene. If this transformation
was synchronous over the length of the boundary then the change may be dated at about
one million Ma.

The proposed plate configuration and relative motions are similar to the continent-
continent convergence of Africa with Eurasia throughout the Middle East and
Mediterranean Sea region (Sengor et al., 1985; Jackson and McKenzie, 1988; England and
Jackson, 1989) (Figure 16). Here, the convergence of Arabia with Eurasia is analogous
to North America-Eurasia convergence. Right-lateral Strike slip and thrusting in the
Zagros Mountains is similar to the seisrrricity of the northern Cherskii Mountains. In
response to generally north-directed compression in the Middle East, Shortening is
accommodated by the extrusion of Turkey to the west, bounded by the North Anatolian

and East Anatolian faults (Sengor et al., 1985). These faults exhibit dominantly strike-slip

Figure 16. Comparison between the Okhotsk plate (left) region and the Middle East
(right). Note that in both regions a block of crusr is escaping laterally in response to
regional crustal shortening. The large arrows indicate the relative motions for the
principal blocks. Small arrows indicate sense of slip along the fault zones. NA = North
America. EU = Eurasia. OK = Okhotsk. AM = Amur. NAF = North Anatolian Fault
zone. EAF = East Anatolian Fault zone. Shading indicates seismically active regions.
Detail of the Middle East taken from Jackson and McKenzie (1988).

41

42

.EH musmam

 

_.. .3 3w _
on Smdm<

Mfla>emza

r

 

«2

e
e‘

A.“ «mm .oms.

__
o .525}. .HW:

 

 

 

i N 0 co

 

—‘
..

I

 

 

 

 

 

 

 

>335... h.42 5...
I! .

o....- uni-Mu .flwmlfifl

EPI-
4‘
a haw-u...-

 

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

oe on. me. on. on.

43

motions: right-lateral on the North Anatolian fault and left-lateral on the East Anatolian
fault. The sense of motion on these faults is identical to the motions observed in the
Sakhalin and southern Cherskii Mountain seismic belts, respectively. In both regions
teleseismic activity is generally confined to zones a few hundred kilometers across, with
motion being accommodated on several subparallel faults.

Rothery and Drury (1984) develop the model of rhombohedral block tectonics to
explain the deformation of the Tibetan Plateau. Deformation within the plateau is
distributed among numerous Strike-slip faults oriented northeast and southeast, defining
rhombohedral blocks up to 200 kilometers across. North-south Shortening in the region
is accommodated by the eastward escape of rhombs along the bounding Strike-slip faults.
The pattern of faults in Northeast Siberia suggests that a similar mechanism may be
aiding the escape of the Okhotsk Plate. Here, the dominant faults are oriented north and
northwest, with shortening directed northeastward. The central portion of the Sea of
Okhotsk is essentially aseismic (disregarding the deep teleseisms related to Pacific
subduction) and is possibly analogous to the megascale rhomb or scholle of Central

Anatolia (Sengor et al., 1985; Jackson and McKenzie, 1988).

CONCLUSIONS

The Sea of Okhotsk and its environs comprise a separate microplate that is escaping
to the southeast in response to the convergence of North America and Eurasia. The
boundaries between the plates are defined by wide (200-300 km) zones of teleseismic and

microseismic activity. Focal mechanisms determined from first morions, body wave

44

modelling, and centroid moment tensor inversions demonstrate that within continental
Asia the North America-Eurasia and Eurasia-Okhotsk boundaries are dominated by right-
lateral transpression, the North America-Okhotsk boundary by left-lateral transpression.
Thrusting and right-lateral strike-slip along Sakhalin Island are due to Okhotsk-Amur
interaction. Microseismicity is generally confined to the northwest corner of the Okhotsk
plate, but does not Show a strong correlation to mapped faults; this may be an artifact of
poor locations due to limited Station distribution. Relative motion between Okhotsk and
the North American and Eurasian plates is taken up along multiple faults, many of which
are visible as lineations in satellite photographs and on topographic maps, subparallel to
the boundaries.

The diffuse nature of the North America-Okhotsk and Eurasia Okhotsk boundaries
suggests that it is probably more correct to refer to a boundary zone, as opposed to a
discrete boundary. Thus, it may be difficult to calculate the NA-OK Euler pole based on
slip vectors from the Cherskii Seismic belt because motion on individual faults may nor
correspond to the true relative motion of the plates due to the rotation of small blocks of
crust within the boundary zone. Similarly, if the northern part of the plate is being
deformed, an Euler pole computed with slip vectors from the southern end of the
Okhotsk-Pacific boundary (Seno et al., 1987) may not accurately reflect observed North
America-Okhotsk motion. Plate inversions based on Slip vectors from the plate margins
may therefore be incapable of satisfying closure for any system including an Okhotsk

plate due to violation of the assumption of rigid-plate motion.

APPENDICES

45
Appendix 1

EVENTS IN THE LAPTEV SEA AND LENA RIVER DELTA

In addition to the earthquakes in northeast Russia, I modelled Six events (Table 2) in
the Lena Delta and Laptev Sea regions, north of my primary study area (Figure 17). Five
of them are natural earthquakes, while the sixth (August 12, 1975) is coincident with a
known Russian peaceful nuclear explosion (PNE), and may represent a combination of
explosive and tectonic energy release. Three of these events (1964, 1980, and 1983) are
of particular interest in that they demonstrate the Strong normal faulting character of

seismicity in the Laptev Sea as far south as 72° N.

Table 2. Focal mechansim data for six earthquakes in the Laptev Sea and Lena River
delta. Symbols and abbreviations are as in Table 1.

46

TABLE 21

 

 

‘ Abbreviations and symbols as in Table 1.

47

N0 YEAR no DY LAT. LON. hlkml Illb H8 ST/DP/RK PL. 1 PL. 2 T-AXIS P-AXIS DATA SOURCE
TIMEW’N) (N) (E)
1963 05 20 72.20 126.25 3.3 5. 260/75/339 260/75 356/70 03/308 25/217 PM '
17 01 40.2
1964 07 21 72.10 130.10 11 5. 339/50/290 339/50 130/45 01/055 75/321 PM '
09 56 17.1
1973 12 15 74.11 147.04 3 4. 4.9 271/77/030 271/77 175/61 29/136 11/040 PM ‘
23 31 43
1975 08 12 70.76 127.12 0.5 5. 330/65/022 330/65 230/71 31/189 04/282 M '
14 59 58
1980 02 01 73.06 122.59 25 5.4 5.3 340/58/320 340/58 094/57 01/037 50/306 PM '
17 30 30.6
1983 06 10 75.53 122.75 22.5 5. 5.4 160/74/235 160/74 047/38 22/275 48/031 PH '
02 13 23

I20 I30

I40

I50

 

LAPTEV SEA

_ 6.
O
V‘

 

 

NEW
SIBERIAN
ISLANDS

 

 

78

76

72

70

Figure 17. Detail of the Laptev Sea region, showing the location and focal mechanism
of the events in Table 2. The number next to each mechanism corresponds to Table 2.
Focal mechanism conventions are as in Figure 3.

48

49
I 983 June 10

One of the most heavily studied events in the Laptev Sea region occurred on June 10,
1983. Previous solutions are dominantly normal faulting, with some strike-slip. The wide
variety of methods used to study this event, including P-wave first motions (Cook, 1988),
P and S-wave inversions (Jemsek et al., 1984; Jemsek et al., 1986), centroid moment
tensors (Dziewonski et al., 1983), and synthetics seismograms (Olson, 1990; this study),
coupled with the close agreement of the solutions yields a high degree of constraint on
the focal mechanism. For this study, nine WWSSN stations were modelled at short and
long-periods (Figures 18 and 19, respectively). The resulting synthetic seismograms are
quite good, and good azimuthal coverage was available. A depth of 22.5 kilometers was
obtained by modelling depth phases. This depth is very similar to the depth of the 1980,
February 1 event located approximately two degrees to the south. The more northerly-
trending plane is chosen as the fault plane to match the strike of grabens mapped by

Avetisov and Guseva (1991).

1980 February I

The February 1, 1980 event occurred near the Lena River delta. P-wave first motions
read from WWSSN seismograms and reported in the ISC Bulletin constrain the
mechanism of this event to include a strong component of normal faulting, although
considerable leeway is allowed in the exact orientation of the nodal planes (Figure 18).
Cook (1988), using surface wave radiation patterns, and Olson (1990), using synthetic

seismogram modelling obtained almost identical solutions of 168/50/336 and 168/50/318

1983 06 10

ALQ spz JER spz

 
  

  
    
     

GDH YKC

‘50:

ALQ LON

GDH spz QUE spz

WWW/1m VVMWWWWVVWN
MECHANISM: 160 74 235
11 DEPTH: 22.5 km
15(1me SOURCE TIME: 1.0 0.0 1.3

CRUST:

2.3 1.3 2.0 1.8
BAG sz LOR SP2 4.? 2.7 2.5 1.6

6.0 3.5 2.7 2.0
6.2 3.6 3.0 21.6

1,, 8.0 4.6 3.3
\3! a B p T
l J

J

 

30 SECONDS

Figure 18. Synthetic seismograms for the June 10, 1983 event. Symbols are as in Figure
4.

50

1983 06 10

YKC lpz JER lpz

BAG lpz LON lpz
JV»?— tr“\/-~——
QUE lpz CHG lpz

WW

I l
60 SECONDS

 

MECHANISM: 160 74 235
DEPTH: 22.5 km

SOURCE TIME: 1.0 0.0 1.3
CRUST:

2.3 1.3 2.0 1.8

4.7 2.7 2.5 1.6

6.0 3.5 2.7 2.0

6.2 3.6 3.0 21.6

8.0 4.6 3.3

a B p T

Figure 19. Long-period synthetic seismograms for the June 10, 1983 event. Symbols are

as in Figure 4.

51

52

respectively. Parfenov et a1. (1987) obtained a mechanism with more of a strike slip
component (191/84/030). A CMT solution by Dziewonski et al. (19882) indicates nearly
pure normal faulting along a northwest trending plane. The intermediate Stress axis of
this mechanism plunges shallowly to the southeast, opposite the northwest-plunging
intermediate axes of Cook (1988) and Olson (1990), although the general trend of the
planes are similar. Short-period synthetic seismograms were computed for six WWSSN
Stations (Figure 20) and, combined with first motion data from the Yakut regional
network, tightly constrain this event. Azimuthal coverage was good with the exception
of the southeastern quadrant. The mechanism shows dominantly normal faulting, with
a component of strike-slip, and is very similar to that of Dziewonski et al. (1988b),
although the intermediate axis plunges more steeply. A depth of 25 kilometers was
constrained by modelling depth phases. This depth is consistent with deep graben
bounding faults identified by Avetisov and Guseva (1991), some of which are interpreted
to reaching depths of up to 30 kilometers. The more northerly trending plane is chosen
as the fault plane, based on the proximity of a north-trending graben (Avetisov and

Guseva, 1991).

1963 May 20

The May 20, 1963 event in the Lena River delta is one of the few strike-slip events
north of the Cherskii Mountains (Figure 21). I reexamined this event using synthetic
seismograms, resulting in a mechanism of 260/75/339; Waveform fits at all Stations were

fair to good. The strike-slip nature of this event is unusual, although its proximity to the

1980 02 01

NUR spz JER spz

“517“” WWW
Jt—---— Jr—-w-v-—

LON spz DUG spz

W MECHANISM: 340 58 320

DEPTH: 25
\fi SOURCE TIME: 1.0 0.0 0.1
CRUST:
2.51.4 2.1 1.0
FVM spz P00 spz 4.0 2.2 2.3 1.0
5.0 2.8 2.5 2.0
6.4 3.7 2.9

\AWWII/ , afipT

 

Figure 20. Synthetic seismograms for the February 1, 1980 event. Symbols are as in
Figure 4.

53

ALQ spz

 

 

 

1963 05 2O

SHI spz

 

1......»
‘A/WWW

NDI spz

1......
114-

 

r—_

30 SECONDS

 

MECHANISM: 260 75 339
DEPTH: 3.3 km

SOURCE TIME: 0.6 0.0 0.3
CRUST:

3.0 1.76 2.1 2.0

4.5 2.60 2.3 8.0

6.2 3.54 2.7

oerT

Figure 21. Synthetic seismograms for the May 20, 1963 event. Symbols are as in Figure

4.

55

inferred pole NA-EU Euler pole and the uncertain stress regime around the pole may
account for this. The north-trending plane follows the Strike of several grabens that are
mapped across the area (Avetisov and Guseva, 1991), making it possible to tentatively

assign this as the fault plane, resulting in right-lateral slip.

1964 July 21

The largest teleseismic event in Buor Khaya Gulf occurred on July 21, 1964 (mb 5.4).
Several focal mechanisms solutions have been determined for this event (table I) and in
general indicate east-west directed extension and nearly pure normal faulting (Cook, 1988;
Olson, 1990). Koz’min (1984) and Imaev and Koz’min (1989), however, suggest a
Strike-slip mechanism with a slight thrust faulting component, and thus place this region
in the compressional regime. A composite mechanism based on microearthquakes
recorded by Avetisov (1991) just south of the epicenter of the 1964 event indicates a
nearly pure thrust mechanism and also suggests that Buor Khaya Gulf is under
compression.

Reread WWSSN and Canadian first motions, covering most of the center of the focal
Sphere are all dilatational and thus indicate a large normal faulting component; these first
motions are internally consistent and clearly disagree with the composite thrust
mechanism of Avetisov (1991). AS long-period body waves proved to be too small in
amplitude to model, synthetic seismograms were calculated using short-period P-wave
records from WWSSN Stations and a crustal model modified slightly from the refraction

results of Kogan (1974) (Figure 22). The results clearly indicate that a normal faulting

1964 O7 21

ATL spz BOZ spz

DUG spz KON spz

5" 151 5 W
l
MECHANISM: 339 50 290
. DEPTH: 11 km
W\W‘“W“ ‘ 55/ ”“U‘W'” ' “ SOURCE TIME: 0.3 0.2 0.3

 

' CRUST:

2.8 1.6 1.8 3.0
NDI spz NUR spz 4.1 2.3 2.3 5.0
704128m0
8.0 4.6 3.3

\IVWM «ASA—W 0‘ B p T
\55‘ [WW \A/xmmww-v—w.

l J
30 SECONDS

Figure 22. Synthetic seismograms for the July 21, 1964 event. Symbols are as in Figure
4.

56

57

solution is preferred. The solution is difficult to constrain to better than about 10° in
strike, based on the relative amplitudes of the major phases in the first 15 seconds, a
north-northwest Striking fault plane is clearly preferred. The depth phases are extremely

well modelled and constrain the focal depth of this event to be about 11 km.

1973 December 15

This event lies near the New Siberian Islands, to the east of the main trend of
epicenters in the Laptev Sea. Previous studies using P-wave first motions from the ISC
Bulletin and reread from Yakut network and WWSSN records suggested a dominantly
thrust mechanism (156 66 124; Fujita, unpublished). I computed Short-period synthetic
seismograms for two Stations, Golden (GOL) and Shiraz (SHI) (Figure 23). Although my
synthetics are of only moderate quality, they clearly Show that GOL must be located near
a nodal plane to minimize the P wave amplitude relative to the depth phase (pP). Many
of the compressions reported in the ISC lie near the b-axis of my mechanism, suggesting

that they may be misidentified depth phases.

1978 August 12

On 12 August, 1975, a mb 5.1 event occurred along the Lena River along Chekanov
Ridge. Based heavily on bulletin reported and WWSSN first motions, Savostin and
Karasik (1981) and Cook (1988) suggested normal faulting mechanisms, however
Koz’min (1984), with additional northeast Siberian data, proposed a thrust faulting

solution. Examination of WWSSN records indicates that no unequivocable dilatations

1973 12 15

GOL spz

iii

SHI spz

MECHANISM:
271 77 30 (Solid)
156 66 124 (Dashed)
DEPTH: 3.0 km
SOURCE TIME: 0.2 0.0 1.5
CRUST:
4.2 2.5 2.3 1.0
5.4 3.2 2.5 1.0
6.0 3.6 3.0 21.6
I J 8.0 4.6 3.3

30 SECONDS a B p T

775

 

Figure 23. Synthetic seismograms for the December 15, 1973 event. Symbols are as in
Figure 4. The top trace in each set is the observed waveform. The middle trace is my
preferred solution (solid line), while the bottom is the thrust mechanism (dashed line).

58

59

have been recorded and the only short-period first motions which are clearly readable are
compressions at several western United States high—gain stations. Since Soviet local
network Stations also reported compressions through northeast Siberia (Koz’min, 1984),
there is some doubt about a normal faulting mechanism for this event.

Short-period synthetic seismograms were calculated for two stations for this event
(Figure 24). Unfortunately, they both plot in essentially the same region of the focal
sphere. Although reasonable fits could be obtained using a double-couple solution, the
focal depth needed to be quite Shallow (~ 0.5 km), and the source-time function was very
Short (~ 0.2 sec). In addition, the synthetics fit fairly well for a isotropic source. Given
that the origin time is essentially on the hour, 15:00:00 + 1.5 seconds UTC according to
the ISC, a peaceful nuclear explosion (PNE) source was suspected, and later confirmed
by Alekseev et al. (1993).

The first motions reported in the ISC bulletin for European and some Soviet stations
to the southwest of the epicenter are dilatational, hence the thrust faulting mechanisms
by Koz’min (1984). My best-fitting synthetics for a double couple source indicate a
dominantly Strike-Slip mechanism, in which European stations would likely be dilatational.
It is possible that the PNE released some tectonic stress thus resulting in first motions
compatible with a double-couple mechanism. 0n the other hand, the initial compression
observed at western US. Stations is weak, and hence only observed clearly at very high

gain Stations.

1975 08 12

DUG spz JCT spz

WWW

ISOTROPIC

W

DOUBLE COUPLE

iili

30 SECONDS

CRUST:

4.0 2.3 2.3 2.0
6.8 3.9 2.8 37.0
8.0 4.6 3.2

or‘BpT

Q

h:

MECHANISM: 330 65 022
SOURCE TIME: 0.1 0.0 0.1

: 0.06 km

11.2.1.1.

W
15W
W
W

0.5 km

:1.0 km

: 5.0 km

10.0 km

Figure 24. Synthetic seismograms for the August 12, 1975 event. Symbols are as in
Figure 4. The synthetics for the isotropic source represent attempts to model the event
as an explosion. The double couple synthetics model the event as a natural earthquake

at different depths.

60

61
Appendix 2

PREVIOUSLY PUBLISHED MECHANISMS

Three of the events I studied in the northern Cherskii Mountains were published in
Riegel et al. (1993). The details of these mechanisms are therefore not repeated in this
text. These events correspond to numbers 25 (Jan. 21, 1976), 11 (Sept. 9, 1968), and 19

(May 18, 1971) of Table 1 and Figure 3.

1976 01 21

GOL spz NDI spz
JCT

W WW

0
W... «MW/(AM— DUG

DUG spz CHG spz

t We

 

MECHANISM: 95 60 12
DEPTH: 10.5 km

JVVVVW’W— SOURCE TIME: 0.3 0.0 0.3
‘ CRUST:

2.5 1.47 2.20 1.5
JCT spz NIL spz 4.2 2.47 2.80 2.2

4.9 2.88 2.75 2.4
6.0 3.53 3.1
55551515055555 5* 5 P T
MW...— W
l J

30 SECONDS

 

 

Sigure 25. Synthetic seismograms for the January 21, 1976 event. Symbols are as in
rgure 4.

62

1968 09 09

COL spz

LOR

COL
ALQ

(f?

LOR spz ALQ spz

MECHANISM: 250 81 25
DEPTH: 4.9 km
SOURCE TIME: 0.3 0.0 0.3
CRUST:

J 3.0 1.71 2.4 3.5

4.5 3.00 2.6 6.5
30 SECONDS 6.0 3.45 2.9

aBpT

%?
7e

—_

Figure 26. Synthetic seismograms for the September 9, 1968 event. Symbols are as in
Figure 4.

63

1971 O5 18

TOL lpz HKC lpz

i?
ii

ESK lpz BAG lpz

MECHANISM: 305 82 6
DEPTH: 13 km

SOURCE TIME: 0.5 4.0 0.5
CRUST:

5.5 3.30 2.9 33

CHG lpz KBS lpz 6.5 3.95 3.3
a B p T

ii
iii?

—
——

60 SECONDS

Figure 27. Synthetic seismograms for the May 18, 1971 event. Symbols are as in Figure
4.

BIBLIOGRAPHY

BIBLIOGRAPHY

Aki, K., and P. G. Richards, Quantitative Seismology Theory and Methods, Vol. I, 574
pp., W. H. Freeman and Co., San Francisco, 1980.

Alekseev, A. A., V. E. Stepanov, V. E. Ushnitskii, G. I. Borisov, V. I. Buturlin, S. V.
Maleev, G. A. Nezhdanov, and B. V. Odinov, Investigations of radiational conditions
in the Bulun region (lower course of the Lena River) Sakha Republic (Yakutia) (in
Russian), in Radiatsionnoe Zagryaznenie Territorii Respubliki Sakha (Yakutiya):
Problemy Radiatsionnoi Bezopasnosti, edited by I. S. Burtsev, pp. 199-214,
Ministerstvo Zdravookhraneniya Respubliki Sakha (Yakutiya), Yakutsk, 1993.

Artarnonov, V. V., and L. V. Mishina, Recording capabilities of the seismic station
network in the northeast USSR (in Russian), in Seismichekie Protsessi na Severa-
Vostoke SSSR, edited by L. I. Izmailov and T. I. Lin’kova, pp. 99-115, SVKNII,
Magadan, 1984.

Artemov, A. V. and I. Y. lvanov, The geologic structure of the Moma continental rift,
Geotectonics, 22, 173-177, 1988.

Aver’yanova, V. N., Seismic Foci in the Far East, 207 pp., Israel Program for Scientific
Translation, Jerusalem, 1973.

Avetisov, G. P. and Y. B. Guseva, Deep Structure of the Lena Delta Region According
to Seismic Data, Sovietskaya Geologiya, (4 ), 73-81, 1991.

Chang, P., W. 8. Jacobs, S. K. Kayanagi, C. k. Lavonne, J. H. Minsch, R. E. Needham,
W. J. Person, B. W. Presgrave, and W. H. Schmieder, Preliminary Determination of
Epicenters, September, 1992, 35 pp., U. S. Geol. Surv., Washington, 1993.

Chapman, M. E., and S. C. Solomon, North American plate boundary in Northeast Asia,
J. Geophys. Res., 81, 921-930, 1976.

Cook, D. B., Seismology and tectonics of the North American plate in the Arctic:
Northeast Siberia and Alaska, Ph. D. dissertation, 250 pp., Michigan State University,
East Lansing, 1988.

Cook, D. B., K. Fujita, and C. A. McMullen, Seismotectonics of northern Yakutia and the
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